Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received January 6, 2018; final manuscript received March 29, 2018; published online May 8, 2018. Assoc. Editor: Ray (Zhenhua) Rui.

The phenomenon of liquefied natural gas (LNG) cargo weathering is considered in terms of the conditions influencing boil-off gas (BOG) rates during the offshore movements and handling of LNG on marine LNG carriers (LNGC), floating storage and regasification unit (FSRU), and floating storage units (FSU). The range of compositions (grades) of commercially traded LNG is significantly broader than the range of compositional changes caused by typical storage times for offshore LNG cargoes. The different nitrogen and natural gas–liquid concentrations of LNG cargoes (i.e., ethane and heavier C2+ components) significantly influence the impacts of weathering and ultimately determine whether the LNG delivered to customers is within sales specifications or not. The BOG from LNG in storage is richer in methane and nitrogen; if nitrogen is present in the LNG, otherwise just richer in methane, than the LNG from which it is derived. This leads to the LNG becoming richer in the C2+ components as ageing progresses. LNG weathering is shown not to play a significant role in the rollover phenomenon of LNG moved and stored offshore, because nitrogen contents are low (typically < 1.0%) and auto-stratification is rarely an issue. LNG stored for long periods on FSU (greater than 8 weeks, or so) experiences significant weathering effects, but most LNG processed by FSRU (and most FSU) has a residence time of less than 30 days or so, in which case weathering has only minor operational impacts. Weathering rates and LNG compositional changes on FSRU for different LNG grades are provided.

Figures

Weathering of LNG stored in a tank leads to a gradual and highly predictable change in composition, with nitrogen and methane contents decreasing and C2+ contents increasing, relative to each other, in the LNG

Typical average weathering tendency curves of mole/mole fractions for commercial LNG based on real observed data and calculated on a 100% mole basis. The curves are derived using REFPROP 8.0 software [26]. Note that the nitrogen curves are plotted at ten times their actual values to be visible on this plot.

Shore-based operations at regasification plants often must adjust gas quality to meet sales-gas specifications. Addition of nitrogen or LPG into the regasified gas, or blending with other natural gas compositions, is carried out to adapt the WI and/or average heating value of the sales gas. The U.S. gas specification shown is a HHV of 40.0 MJ/m3 (equivalent to 1075 Btu/scf), and a nitrogen dilution limit of 3% applies. The UK gas specification shown is a WI of 51.41 MJ/m3, and a nitrogen dilution limit is 5% applies. Heating values and Wobbe Indices plotted here are for the average of lower and HHV.

Stratified LNG in tanks (top) equilibrating to single layers (bottom) following rollover. Convection currents, BOG formation, mass transfer, and heat sources drive the rollover phenomenon, with LNG weathering of the top layer making a minor contribution. See also Kulitsa and Wood [32].

Schematic showing the same cargo quantity on FSRU and FSU, but distributed differently resulting in different ratios of heating surface area to LNG mass in each case (higher ratio on FSRU). Consequently, the BOG rate is higher on the FSRU in this case. However, because of the distinct operating patterns of FSRU (i.e., LNG is likely to be onboard FSRU for shorter time periods than on FSU, and the FSRU uses it recondenser to process BOG), the LNG weathering effect experienced on FSRU is likely to be less than on FSU.

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